1. Field of the Invention
The present invention relates to a projection exposure apparatus and projection optical system being part thereof as are used in photolithography for manufacturing semiconductor or liquid crystal devices, for example.
2. Description of Related Art
Optical lithography has become a key technology for the fabrication of electrical and optical integrated circuits of various kinds. Since the smallness of such circuits is mainly determined by the imaging systems of the lithographic devices used in the fabrication process, considerable efforts have been spent on improving the resolution of these imaging systems.
One way to achieve higher imaging resolutions in such systems is to use shorter wavelengths. At present, commercially available projection exposure systems with the highest resolutions use UV light of wavelengths 193 nm or 157 nm. Research and development activities already consider to enter the domain of extreme ultraviolet radiation in which “soft” X-rays are used having wavelengths of about 10 nm to 30 nm. One of the main problems encountered when using such small wavelengths is the fact that conventional optical refractive components such as lenses are almost completely opaque in this wavelength range. Future X-ray projection systems are therefore likely to contain only reflective optical components, i.e. mirrors of various kinds.
However, due to the high energy of electromagnetic radiation in this extreme UV domain, heating of the mirrors is of major concern. The mirrors envisaged for the application in projection optical systems are made of a mirror support on which a layered stack of dielectrics is deposited forming a reflective layer. Although reflectivity of this layer may be well beyond 50%, the absorbed radiation dissipated on the surface of the mirror amounts to a considerable amount of heat during projection. This heat results in a temperature increase of the reflective layer and also of the mirror support that, hence, changes its shape due to thermal deformation.
In order to avoid imaging aberrations caused by deformations of the mirrors, it has been proposed to employ a metal as material for the mirror supports, thereby increasing the heat abduct from the mirror, and to use active cooling for these metal mirror supports, see for example EP 0 955 565 A1. Active cooling, however, increases system complexity and costs. Systems without active cooling cannot sufficiently eliminate the image deteriorations. Apart from that, metal surfaces have to be further processed before the reflective stack of layers can be deposited thereon.
Another approach for solving the mirror heating problem has been described in DE 100 40 998 A1, corresponding to U.S. Ser. No. 09/934,252. According to this known approach, a change of the imaging properties of a lens due to illumination-induced heating is at least partly compensated by an opposite illumination-induced change of the imaging properties of a mirror. This approach is based on the observation that changes in the radius of curvature of optical components, for example a reduction in the radius of curvature of a concave optical surface, have opposite effects on the optical imaging properties of said surface depending on whether the surface is a reflecting or a refractive one. In the projection exposure systems considered herein using wavelengths in the extreme ultraviolet, however, there are usually no refractive optical components that could be used for compensating thermally induced aberrations.
According to another approach known from WO 01/08163 A1, the operating temperatures of the mirror supports are determined beforehand. Then the coefficients of thermal expansion of the mirrors are adjusted such that each mirror support has, at its respective operating temperature, a coefficient of thermal expansion centered about 0. Adjusting the coefficients of thermal expansion is achieved by controlled tuning of a Ti dopant concentration in high purity SiO2 glass. However, such materials are expensive and considerably increase the overall system cost.
Therefore there is a need for a projection exposure apparatus comprising mirrors in which the amount of thermally induced aberrations is reduced, but without increasing the system complexity by active cooling devices.